1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus. The optical scanning apparatus is considered particularly suitable for an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, and a multifunction printer (MFP) configured to perform an electrophotography process.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a LBP, MFP and digital copying machine employs an optical scanning apparatus. In such an optical scanning apparatus, light fluxes that are light-modulated and emitted from a light source (laser or the like) in response to an image signal are periodically deflected by a deflection unit including, for example, a rotational polygon mirror (polygon mirror). Then, the deflected light fluxes are converged in the form of a spot on a photosensitive surface (surface to be scanned) of a photosensitive member (image bearing member) by an imaging optical system having fθ characteristics, and then the photosensitive surface is optically scanned to record an image.
In a color image forming apparatus, an optical scanning apparatus having a single deflection unit is shared by a plurality of photosensitive members and a plurality of light fluxes is caused to obliquely enter a same deflection surface of the deflection unit within a sub-scanning cross section to simultaneously scan a plurality of photosensitive surfaces. Japanese Patent Application Laid-Open No. 2008-15139 discusses an arrangement in which light fluxes deflected by the same deflection surface of a deflection unit are respectively guided to corresponding photosensitive surfaces via a single image forming lens to simplify optical members and reduce the size of the entire optical system.
However, the arrangement discussed in Japanese Patent Application Laid-Open No. 2008-15139 has a problem in that a part of the light fluxes is reflected by a surface of the image forming lens and the deflection surface to become unnecessary light (ghost light) entering a photosensitive surface that is to be scanned by another light flux. Furthermore, an arrangement in which imaging optical systems are provided on both sides of a deflection unit to sandwich the deflection unit has a problem that light reflected by a lens surface of one of the imaging optical systems enters as unnecessary light (ghost light) into another imaging optical system across the deflection unit.